This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
External gear machines (EGMs) are used as primary flow supply unit in many applications such as fuel injection systems, small mobile applications such as micro-excavators, turf, and gardening machines. EGMs are also used in fixed applications such as hydraulic presses and forming machines. EGMs also find applications in auxiliary systems such as hydraulic power steering, fan drive systems and as charge pump in hydrostatic transmissions.
Referring to FIG. 1A, a perspective view of an example of an EGM 10 is depicted. The EGM 10 includes a housing 12, a drive gear 14, which drives a slave gear 16, both disposed inside the housing 12. The drive gear 14 and the slave gear 16 are supported by bushings 18 inside the housing 12. The drive gear 14 and the slave gear 16 are coupled together in a mesh zone where a plurality of their respective teeth comes into contact with each other. Tooth space volumes between any two consecutive teeth of the drive gear 14 and any two consecutive teeth of the slave gear 16 pick up fluid and deliver fluid as the teeth rotate about the housing 12. Specifically, in the mesh zone the tooth space volumes initially decrease as the respective teeth come into contact with each other and increase as the teeth come apart from each other. As the tooth space volume decreases, fluid pressure increases, causing ejection of fluid through an outlet 22 at an output pressure. Similarly, as the volume increases, the pressure decreases causing suction of fluid from the inlet 24 at an inlet pressure. End caps 26 and 28 enclose the housing 12, where the end cap 26 provides a journal support for the drive gear 14.
Referring to FIG. 1B, a perspective view of an example of the bushing 18 is provided. Fluid is communicated via an outlet fluid communication channel in the form of a groove 30 from the varying spaces between the teeth in the mesh zone to the outlet; and similarly fluid is communicated via an inlet fluid communication channel in the form of a groove 32 to the varying spaces between the teeth in the mesh zone from the inlet. Therefore, grooves permit to utilize the full volumetric capacity of the unit, avoiding localized pressure peaks and fluid cavitation. In a pressure compensated EGM, such as the one represented in FIG. 1A, these grooves are realized in the bushings 18 (FIG. 1B)—popularly known by the names of bearing blocks and pressure/thrust plates—used to realize optimal sealing of the tooth space volumes through a proper lubricating fluid film even at high operating pressures. In non-compensated EGM, these grooves are machined in the pump housing.
The above described principle of operation of an EGM makes these units inherently fixed displacement. This inability of adapting the fluid displaced per every revolution on the basis of user's requests makes EGMs unsuitable for applications in energy efficient system layout configurations which characterize many fluid power applications. In these system configurations, variable displacement units can offer energy saving even greater than 50% compared to solutions based on fixed displacement units.
These factors have driven significant research towards the definition of a working concept for variable displacement EGMs. Past effort can be broadly categorized into two different sets of solutions: the first set of solutions consists of changing the meshing length of the gears. Several patent references describe different solutions for this idea, by moving the gears axially (US2001024618, EP0478514, US2008044308, and US2002104313). The second set of solutions consists in changing the inter-axis distance between the gears, thereby affecting the meshing area of the gears as provided in at least two patent references (CN85109203 and GB968998). However, each of these solutions introduces significant technological challenges, such as complexity, and has not resulted in successful commercialization. In fact, several major issues have to be faced to implement a viable and cost effective solution to move the gears, which are the most mechanically loaded parts of the machine, requiring at the same time good sealing and smooth transmission of power between the gears.
Efforts to obtain variable flow supply units were also made at system level; in particular, solutions that combine fixed displacement pumps with fast switching valves controlled in pulse width modulation (PWM) to obtain a variable output flow were proposed by several researchers. Despite the theoretical validity of these so called “virtually variable displacement” solutions, their application in real systems is hampered by the limited time response of electromechanical valves as well as compatibility issues of current fixed displacement pumps with the introduction of severe pressure pulsations.
There is, therefore an unmet need for a novel approach to provide variable flow at low and high pressures in gear pumps.